An optical fiber has become known, for example, from U.S. Publication No. 2005/0117860 A.
Optical fibers are used in conjunction with high-power lasers as optical waveguides, in order to carry the laser radiation from the laser over a path to an application, e.g., to a material processing device. Optical waveguides (such as optical fibers) for the transmission of laser radiation in the kilowatt-power region usually consist of a quartz glass core (optical fiber core) and a quartz glass cladding (optical fiber cladding), which due to suitable doping or microstructuring has a lower (effective) refractive index than the optical fiber core. The core carries laser radiation up to a maximum acceptance angle by means of total internal reflection. The cladding is not used for light transmission, but provides the lower refractive index necessary for the total internal reflection. To protect the optical fiber, a protective sheath made of a flexible material (e.g., silicon or acrylate) is applied to the quartz glass cladding. The protective sheath, in general, has a lower refractive index than the quartz glass cladding and partially absorbs the laser radiation.
Optical fibers with a microstructured quartz glass cladding, which are also referred to as “photonic crystal fibers”, have a quartz glass cladding, which contains longitudinal, air-filled capillaries, in order to reduce the effective refractive index of the quartz glass cladding. The capillaries extend parallel to the fiber core. The spacing and diameters of the capillaries determine the numerical aperture of the fiber core: the larger the diameter of the capillaries and the closer together they are arranged, the higher is the numerical aperture of the fiber core. The numerical aperture of the fiber core influences the beam quality of the transmitted radiation. A recent design for optical fibers for the transmission of laser radiation of high (peak-pulse) power is offered by hollow fibers, which instead of a solid quartz glass core have a hollow, air-filled core. In these hollow fibers the core is also surrounded by a capillary-threaded quartz glass cladding.
To couple into the optical waveguide fiber, a laser beam is focused for example, on the input end of the fiber. When using this method, part of the laser radiation can also enter the cladding region of the fiber. This occurs, in particular, during the necessary mutual adjustment of laser beam and optical fiber. Equally, radiation power can enter the cladding region due to backscattering and reflection from the workpiece being processed. This proportion of the laser radiation entering the fiber cladding can also be conveyed by total reflection outside the fiber core, if the optical fiber is surrounded by a flexible protective sheath that has a lower refractive index than that of the cladding region. If, e.g. at fiber bends or spliced connections, the total reflection of the cladding region is disturbed, then laser radiation can escape from the quartz glass cladding into the protective sheath, or pass through it. Due to radiation absorption in the protective sheath, this can be heated and destroyed. The radiation not carried in the fiber core can lead to damage to the end regions of the fiber, in particular. Furthermore, the beam quality of the transmitted laser radiation is worsened, which can adversely affect following optical components and the processing quality.
In the optical fiber known from the above mentioned U.S. Publication No. 2005/0117860 A, the capillary-free longitudinal section forms a side window, in order to couple light into the inner fiber cladding or out of the inner fiber cladding. The capillary-free longitudinal section can be produced by lengthening the optical fiber, causing the capillaries to collapse in that area and at the same time causing the fiber core to become tapered. In the tapered fiber core section, higher-order modes propagating in the fiber core are eliminated, that is, the tapered fiber core section acts as a mode filter, where the eliminated higher-order modes propagate in the inner fiber cladding.
From U.S. Pat. No. 4,637,686, an optical fiber is known with an outer fiber cladding surrounding the inner fiber cladding, which over its entire length has substances for the dispersal or absorption of light propagating in the fiber cladding. The outer fiber cladding has a higher refractive index than the inner fiber cladding, so that light in the inner fiber cladding is not guided by total reflection and enters the outer fiber cladding, where after only a short distance it is dispersed or absorbed by the substances.
From U.S. Publication No. 2004/0071420 A, a further optical fiber is known, which has one or more zones with constantly varying refractive index, so that light propagating in the fiber core can escape into the fiber cladding.
Finally, EP 1 213 594 A discloses another optical fiber, the fiber cladding of which has scattering elements.